The present invention is directed towards a method and apparatus for modifying a video signal and, more particularly, to a method and apparatus wherein the vertical synchronization interval of the video signal is modified such that a video picture can be reproduced directly from the modified video signal but cannot be reproduced from a recording of the modified video signal.
With the recent introduction of video signal recording systems, such as video tape recorders (VTR), there has evolved a substantial market for copyrighted video tapes for home use. By way of example, many popular movies have been recorded on the video tapes and sold at substantial prices to the home video recorder market. As a result of the relatively high cost of these video tapes, there has been a great incentive to illegally pirate these tapes by playing them on a first VTR and recording the output signal generated by the first VTR on a blank tape contained in a second VTR.
In order to protect their investment, recorders of the copyrighted tapes have attempted to modify their tapes in such a manner that they cannot be successfully reproduced. The present invention achieves this result by modifying the video signal contained on the copyrighted tapes in such a manner that they cannot be successfully reproduced on a standard VTR. Since the video signal itself is being modified, the invention may also be successfully utilized to prevent the unauthorized recording of broadcasted television programs.
A standard video tape includes a plurality of field tracks which are evenly spaced along the length of the video tape. Each track includes all the video information required for a single TV field. The beginning of each track includes a vertical blanking interval which is followed by all of the video information for a single TV field. The vertical blanking interval includes a serrated vertical synch pulse formed of a plurality of sub-pulses which are utilized by the vertical oscillator located in a television monitor to properly locate the beginning of each frame of information at the top to the TV picture. This information is essential to the television monitor for proper operation thereof.
In addition to the video tracks, a plurality of control pulse indicator marks are magnetically imprinted on the video tape. The control pulse indicator marks define a control track which is used by a servo mechanism in the VTR to maintain synchronism between the linear velocity of the video tape and the rotational velocity of the VTR tape head (this head contains a pair of mechanically coupled magnetic heads) when the VTR is operated in the playback mode. The control pulses are normally equal in number to the number of video field tracks and have a predetermined spaced relationship therewith. The servo mechanism of the VTR adjusts the rotational velocity of the VTR tape head as a function of the linear velocity of the control pulse indicator marks past a control pulse detector forming part of the servo mechanism. This assures proper frequency adjustment between the tape head and the video tracks. Proper phase adjustment is normally maintained by the prdetermined spacing between the video tracks and control pulse indicator marks. A manual phase control adjustment is provided for this instance where phase adjustment is lost due to machine to machine variations, tape stretching and other causes.
In accordance with the foregoing, the rotational velocity of the tape head and the linear velocity of the tape are normally synchronized to ensure that the tape head scans one video track each time it sweeps past the tape. If the control pulses are removed from the video tape, the tape head of the VTR will not stay in synchronism with the movement of the intermittently intelligible signal on a standard TV receiver. The foregoing fact has previously been recognized by the prior art as exemplified in U.S. Pat. No. 4,100,575.
A VTR is capable of operating in two modes: a playback mode and a record mode. When operating in a playback mode, the VTR monitors the frequency of the control pulses passing a first tape head adapted to scan the control pulse track and adjust the velocity of the VTR tape head scanning the video track so as to cause the VTR tape heads to remain in synchronism with the video tracks. When operating in the record mode, the VTR accepts an incoming video signal (containing both the desired video information and synchronizing pulses) and records both the video signal (including the vertical synch pulses) on the video tracks and the control pulses on the control track. The VTR records a control pulse on the control track of the video tape each time it detects the vertical synchronization pulses in the incoming video signal.
In a typical video signal, the vertical blanking interval includes a serrated vertical synch pulse (containing six sub-pulses) of a standard width (27.3 .mu.sec). As noted above, these sub-pulses are detected by the vertical deflection circuit of a standard television monitor to control the vertical positioning of the video signal. To this end, most vertical deflection circuits include an integrating circuit which integrates the serrated vertical synch pulse. When the output of the integrating circuit reaches a predetermined level, the vertical deflection circuit "recognizes" the presence of a vertical synchronization pulse and causes the vertical oscillator to return the scanning beam to the top of the video tube, thereby maintaining the vertical oscillator of the TV monitor in synchronism with the vertical oscillator of the television camera.
The serrated vertical synch pulse is also used by the VTR to synchronize the motion of its magnetic tape head and the video tape when the VTR is operating in the record mode. Like the television monitor, the VTR normally includes an integrating circuit which integrates the serrated vertical signal pulse. When the output of the integrator reaches a predetermined level, the VTR "recognizes" the presence of a vertical synch signal and records a control pulse on the control track of the video tape. Significantly, the sensitivity of the video monitor integrating circuit is much greater than that of the VTR integrating circuit. Thus, while the time constant of the television monitor integrating circuit is relatively short, that of the VTR integrating circuit is relatively long. This difference in sensitivity can be used to modify the serrated vertical synch pulse in a manner which makes the pulse "visible" (i.e., detectable) to the video monitor but "invisible" (i.e., not detectable) to the VTR.
One method for modifying the serrated vertical synch pulse is disclosed in U.S. Pat. No. 4,100,575. In this patent, the vertical synch pulse is modified to include only 2 sub-pulses so as to make the modified pulse visible to the television monitor but invisible to the VTR. Another method for modifying the serrated vertical synch pulses is to maintain all 6 sub-pulses but to reduce their width (e.g., to 13.7 .mu.sec). Yet another method would be to reduce the height of the sub-pulses or to cause certain sub-pulses to pulse in the negative direction. For the purpose of this invention, any method which will modify the vertical synch pulses in a manner in which they can be detected by the video monitor but not detected by the VTR can be used.
In order to prevent the unauthorized copying of video information on a VTR operating in the record mode, the prior art, as exemplified by U.S. Pat. No. 4,100,575, modifies the video signal in such a manner that each (or at least every other) serrated vertical synch pulse contains only two sub-pulses. As a result, the video signal contains sufficient information to enable the TV monitor to detect the vertical blanking interval (and thereby maintain proper vertical alignment of the video signal) but does not contain sufficient information to enable the VTR to detect the vertical blanking interval (and thereby record a control pulse on the video tape). As a result, any tape recording reproduced on the VTR receiving the modified video signal does not contain the standard control pulses and cannot be used to successfully reproduce a video picture on a standard television monitor when placed in a VTR operating in the playback mode.